I just wanted to comment on a very common misconception about the 4th valve operation on a 4200XL.
The valve is controlled and adjusted with the dial just above the left stick. Once you set it, that's it. The valve does not move while running stick boom and bucket. There is no mix on this set up.
Knowing that, I have no idea why the sound is changing considering that ch is constant voltage?
Sorry I can't be more help.
Excavators don't really rev up and down at all. Just lug a little when working hard. Maybe use a toggle switch function to rev up and idle down when not digging?
Oh, PS. I'm a RC guru wanna be
Mario